This disclosure concerns an invention relating generally to post-construction reinforcement of structures (such as buildings, bridges, dams, and the like), and more specifically to structural reinforcements externally affixed to preexisting structures.
In recent years there has been an increase in the use of lightweight, nonmetallic, fiber reinforced composite materials to repair and strengthen concrete structures. A common repair method is to adhesively bond strips of thin composite laminates, also known as fiber reinforced polymer (FRP) strips, to the surfaces of reinforced concrete beams or slabs to increase their capacity. Typically these composite strips are attached to the undersides of the beams/slabs to increase the flexural capacity of the reinforced concrete element. The method used to strengthen concrete beams with composite strips is similar to one that has been used with some popularity since the mid-1970""s, particularly in Europe, to repair concrete beams with steel plates. In one popular method, a composite strip manufactured by the Sika Corporation (Lyndhurst, N.J., USA) is bonded to the concrete surface with a room temperature curing two-part epoxy adhesive. This method is time-consuming since it can take days per application to sandblast, clean, and smooth the concrete so that it is suitable for bonding. Additionally, the two-part epoxy system must be mixed in a precisely controlled fashion and applied in a careful manner to produce a good bond line. Following the application of the adhesive, the composite strip must be left for at least a day, and often the adhesive will not reach design strength for approximately a week.
Other systems (e.g., one promoted by Master Builders Inc., Cleveland, Ohio USA) make use of preformed fiber fabrics and apply the epoxy resin system to the fabric and to the concrete substrate simultaneously. These systems require the same careful and time-consuming preparation and curing as in the case of bonding a prefabricated composite strip to the concrete.
In situations where it is necessary to make extremely rapid repairs to structures, e.g., where military operations require rapid repairs of bridges, or disaster relief efforts require that wall or ceiling beams be quickly reinforced, the foregoing adhesive bonding methods are clearly insufficient owing to the curing time needed for the adhesive. Thus, there is interest in developing mechanical attachments for reinforcing strips that would replace the time-consuming bonding methods. Prior methods of structural reinforcement use externally affixed xe2x80x9ctendonsxe2x80x9d, generally made of steel, which are bolted to the structure at their ends. These are generally unsuitable for rapid repairs owing to the time needed to suitably affix the tendons to the structure, and the size and weight of the tendons generates additional problems because they are difficult to transport and install by military and/or emergency personnel with minimal tools and manpower. The use of composite strips in place of metal tendons would ease transportability and weight concerns, but the use of mechanically attached composite strips has not been accepted because the stress concentration points created by the fasteners tends to greatly decrease the strength of the strips. The high loads at the fastener holes in the strips cause ripping, which propagates through the strips until failure occurs. Thus, mechanical attachment of composite strips has thus far been primarily limited to the use of anchorages (e.g., anchor bolts or cover plates) at the ends of adhered composite strips, not to serve as the primary load transfer mechanism between the concrete and the composite strip, but to prevent catastrophic brittle failure of adhered strips when the adhesive bond separates from the underlying structure. Similar mechanical anchorages have been used with epoxy-bonded steel plates to prevent failure from the plates peeling from the concrete.
The invention, which is defined by the claims set forth at the end of this document, is directed to methods and apparata which at least partially alleviate the aforementioned problems. A basic understanding of some of the preferred features of the invention can be attained from a review of the following brief summary of the invention, with more details being provided elsewhere in this document.
Preferred versions of the invention involve a structural reinforcing strip which is affixed to a structure to be reinforced by the use of several fasteners which extend through the strip and into the structure. The reinforcing strip preferably includes elongated continuous parallel fibers which have lengths extending along the length of the strip, and nondirectional fibers distributed transversely across the strip, with a polymer matrix affixing the parallel and nondirectional fibers. The parallel fibers are preferably provided in multi-fiber bundles (e.g., rovings or tows) which are discretely spaced transversely across the strip. The nondirectional fibers, which may be defined by a nonwoven mat provided within the polymer matrix, are preferably distributed at least substantially uniformly across the strip. The strip may be dimensioned so that it can be coiled into a roll for easy transport, and it may then be uncoiled and cut to length at the site at which it is to be used. The cut strip may then be placed on the structure to be reinforced, and may be attached thereon by actuating a common powder-actuated fastener gun to send fasteners through the strip and into the structure. If desired, pilot holes for the fasteners may be pre-drilled into the structure prior to insertion of the fasteners through the strip and structure to diminish potential damage to the underlying structure (e.g. spalling where the structure is made of concrete, or cracking where the structure is made of wood or other materials). Additionally, compressible cushions (such as rubber/neoprene washers) may be provided between the fasteners and the strips prior to inserting the fastener through the strip, so that the fastener heads (assuming they are present) will bear against the cushion, rather than directly against the strip. Adhesive may also be applied between the strip and the surface of the structure prior to attaching the strip thereon.
A strip as previously described, being affixed to a structure in the foregoing fashion, is believed to provide several advantages that were not previously fully realized in prior structural reinforcement methods and apparata.
Initially, the invention is well suited for use in rapid structural repairs because the strips (or coiled strips) are easily carried by a single person, easily cut by battery-operated tools suitable for field use, and easily affixed to structures by use of portable fastener guns which allow fastening without the need for pre-forming holes in the strips. The invention is therefore particularly useful in field conditions wherein manpower, power supplies, lifting equipment, and other resources are scarce or difficult to access. Since the strips may be installed by a single person with no or minimal prior training, the invention is extremely useful in cases of disaster, where emergency personnel may need to rapidly perform unfamiliar structural reinforcement tasks without education or supervision. Since no time-consuming adhesive curing is required, the invention is readily usable upon installation, which further enhances its utility where time is short.
Further, the strips are believed to provide superior strength per unit size and weight owing to their unique structure, which is particularly suited for usage with fasteners. Ordinarily, the stress concentrations caused by the use of fasteners with composite reinforcing strips results in splitting failure of the strips. Such failure may be exacerbated where fasteners are driven into strips wherein fibers are oriented in predetermined directions, since the fastener driving force, or the bearing stress exerted by the fastener on the strip, may cause fractures to occur along planes parallel to the fibers (regardless of whether they are parallel to the axes of the strips or at other orientations, and whether the fibers are unidirectional or multidirectional). By using nondirectional fibers, no well-defined fracture planes are provided, and strip fractures are less likely to form and propagate upon insertion of the fastener. The inclusion of fibers oriented parallel with the lengths of the strips then increases the load-bearing capacity of the strips, particularly since the fastener loading on the nondirectional fibers is transmitted to the parallel fibers. Additionally, the nondirectional fibers transmit the fastener loads to the parallel fibers, and thereby distribute forces over a larger area for greater strength Testing has demonstrated that when structures reinforced with the strips fail, unless catastrophic failure first occurs in the underlying structure (e.g., the position of the structure underlying the fasteners breaks away), the strips impart greater ductility to the structure and allow greater deflection prior to ultimate failure. This provides more time for warning and implementation of additional reinforcement measures, and is thereby much safer than the catastrophic failure experienced with many prior reinforcing strips.
Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.